Mine wasn't a virus per se...
A friend of mine ran a local BBS & kept crowing about how he had "secured it tighter than a virgin's morals".
He challenged me to find some way of infecting the BBS, and I accepted.
The result was a batch file that typed the ASCII code for a Space into a text file, copied that first text file into a second, then kept concatenating the files to the end of each other until the files were so huge that they filled the HDD.
I had to boot to a floppy, delete the second text file, clear the temp files, & reboot to the HDD, at which point I used PKZip to max compress the remaining text file down to nothing.
Once the zip file was made, I ran the batch file again.
I kept it up, creating the massive "empty" text files, rebooting to a floppy, deleting one of them & clearing the cache, and rebooting to add the resulting text file to the zip file.
By adding to rather than overwriting the compressed file, I ended up with a zip file nearly half a megabyte in size, in an era when 10Megabyte hard drives were still considered exclusive to data centers & server farms.
I changed the name of the compressed file (inside the zip container) to File_ID.Diz, this being the Description file that most BBSi would extract to use to post the description of the uploaded file to automated listings.
After adding a legit file inside to make it appear that the zip was also legit, I uploaded the whole thing to my friend's BBS.
The system promptly crashed when it attempted to uncompress the "tiny" description file, only to have what would normally uncompress to maybe a few Kb for a really detailed description, into something that spanned *GigaBytes*.
He didn't have the computational horsepower to handle the job, didn't have the HDD space to store it, and the board went down faster than a crack whore Jonesin for a fix.
He called me up screaming that I'd killed his machine, and I had to take repeated deep breaths to control the laughter.
I explained what I'd done, how I'd done it, & the simple steps needed to recover from the "virus" (reboot to floppy, delete the text file, flush the cache, & reboot to the HDD), and he promised to dismember me the next time I came over.
I made up for it by bringing over a pizza & case of soda, and he said he'd let me live.
So I never got into programming virii, I merely figured out ways to use systems against themselves, so a run away process, RAM & HDD gobbling "clean" program could do what Anti Virus software would have otherwise detected & quarrentined first.
"Oops, I'm sorry, did your uber fast 8088 with a full Meg of RAM and 5Meg Hard Drive suddenly shit itself? Good thing that wasn't a virus I uploaded to your secure system, huh?"
*Thumbs in ears, feisty raspberry & happy hooty monkey noises*